1. Technical Field of the Invention
This invention relates generally to the field of communications antennas. More specifically the present invention relates to reconfigurable feed networks for electronically beam-steered planar antenna structures.
2. Background
The development of antennas for use on moving platforms such as aircraft and ground vehicles has not been particularly difficult for low frequency applications where near-omnidirectional antenna beam patterns provide sufficient radio frequency (RF) gain. However, at higher frequencies an air or ground vehicle antenna must possess a degree of spatial directionality to achieve sufficient gain to close transmit and receive communications links.
Spatially-directional antennas used in air and ground vehicle applications must also have beam steering capabilities in order to maintain line-of-sight communications. Where the dynamics are not too great, beam steering on moving platforms has been accomplished by mechanically steering means. However, when dynamics are high, electronic beam phase-shift steering is the only means that will suffice.
When airborne antenna applications will have an adverse impact on aerodynamics planar, electronically phase-shift steered antennas represent the only viable solution because they afford integration into the airframe with minimal disturbance to airflow. Conformal antennas provide the ultimate solution to integration into an airframe because conformal arrays can be shaped to match portions of an aircraft such as wing leading edges. The application of multiple conformal arrays also relaxes the requirements for phase steering because at any given time the conformal array pointed being oriented nearest to boresight can be selected to carry the communications link.
Moreover, because antennas are generally designed to operate at a given relatively narrow frequency band, by design, their operational frequency range is generally fixed. Wide bandwidth antennas solve the problem of having to integrate a separate system of antenna arrays into an aircraft for each frequency band of interest. To the extent that a single antenna array can be reconfigured in real time to support multiple frequency bands of operation, the better in terms of power, weight, and space.
What is needed therefore is a communications antenna system and structure that provides real time control over electronic beam steering and operational frequency band, while possessing a simple planar structure with adaptability to conformal integration with a host platform.
3. The Prior Art
Non-patent reference to Maloney et al [1] discloses a method that addresses the physical size of antenna arrays by employing “fragmented aperture” techniques to provide controlled reception pattern antenna arrays having one-quarter the footprint of conventional arrays. Finite difference time domain code is applied to computationally model the fragmented aperture for optimization over gain, steering, bandwidth, and physical dimension. While apparently successful in reducing array size for a given bandwidth, the fragmented aperture technique does not provide the flexibility afforded by real time reconfigurability of either parameter.
Non-patent reference to Georgia Institute of Technology [2] discloses a method that apparently creates a bandwidth of 33-to-1 in a planar antenna array of given size by exploiting the properties of mutual coupling between antenna elements. However, nothing in this reference indicates that mutual coupling, and therefore bandwidth, may be varied in real time or that the mutual coupling properties are not dependent upon antenna structure planarity, so as to make amenable to conformal applications.
Non-patent reference to Syntonics, LLC entitled Pixel-Addressable Reconfigurable Conformal Antenna (PARCA Software Defined Antenna™) [3] discloses a method for dynamically adjusting the operating frequency, beamwidth, and polarization while transmitting. The PARCA™ employs movable, millimeter-scale, microstrip transmission line pixels with uniform size and dimension to create a rapidly, pixel-by-pixel, changeable antenna pattern upon command. While this reference apparently provides real time control of beam steering and bandwidth with adaptability to conformal applications, the method of operation requires the physical movement of microstrip pixels into and out of alignment with the radiating elements' plane, with no disclosed means for providing such movement.
Non-patent reference to Pringle et al [4] discloses a reconfigurable antenna array employing field effect transistors (FETs) as switches that interconnect radiating patches on the antenna's surface. To reduce control signal routing, the FETs are overlaid by a corresponding array of light emitting diodes (LEDs). The LED light illuminates a photo-detector in parallel with the gate-source junction of the FET, causing the gate source voltage to drop thereby opening the FET switch so as to connect an adjacent radiating patch. As many radiating patches as are interconnected will define the instant configuration of the antenna. While this reference represents an advancement in the state-of-the-art of reconfigurable antennas it has not overcome the necessary complexity of routing bias voltages to each and every FET, nor the associated power consumption. Additionally, the reference discloses that FET switches cause signal losses at microwave frequencies and that the metallic bias lines to each FET introduce scattering that distorts the antenna pattern.
What the prior art fails to provide and what is needed, therefore, is an antenna which (1.) is steerable and reconfigurable in terms of operating bandwidth and radiation pattern; (2.) planarized yet suitable for conformal applications; and (3.) is minimally dependent upon active circuitry and physical and electrical interconnections that create signal loss and antenna distortion.